Nitride semiconductor devices are characteristic in terms of their higher saturation electron velocities and wider band gaps. Efforts for developing devices with higher withstanding voltages and higher outputs by exploiting such characteristics have been made actively.
Particularly, one type of nitride semiconductor devices used for such devices with higher withstanding voltages and higher outputs is a high electron mobility transistor (HEMT).
For example, an AlGaN/GaN-HEMT has been developed, which has a HEMT structure wherein GaN is employed as an electron transit layer, AlGaN is employed as an electron supply layer, and an AlGaN layer is staked on a GaN layer. In an AlGaN/GaN-HEMT, strain is generated in AlGaN due to the difference in the lattice constants of AlGaN and GaN, which induces piezo polarization. High density two-dimensional electron gas (2DEG) is generated by the piezo polarization and the spontaneous polarization of AlGaN. In this manner, an AlGaN/GaN-HEMT can provide a device with a higher withstanding voltage and a higher output.
Such an AlGaN/GaN-HEMT, however, exhibits higher contact resistances of a source electrode and a drain electrode, due to the HEMT structure stacking an AlGaN layer on a GaN layer.
Against such a background, an AlGaN/GaN-HEMT with reduced contact resistances has been proposed, which has an inverted HEMT structure having a nitride-polar surface and a GaN layer stacked on an AlGaN layer.